Organic Chemistry I

For the same atom, an sp hybridized atom is more electronegative than sp
2
hybridized atom, which is
more electronegative than sp
3
hybridized atom.
3.4 Structural Effects on Acidity and Basicity | 105

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3.5 Lewis Acids and Lewis Bases
The Brønsted-Lowry definition works well for the reactions we learned so far, however it also limits the scope of acid-
base reactions in a way where the proton H
+
must be involved. Lewis acids and Lewis bases are defined in a more
inclusive way that was first introduced by G. N. Lewis in 1923.
Lewis Acid
: a species that can accept an electron pair;
Lewis Base
: a species that can donate an electron pair.
All Brønsted-Lowry acids and bases fit into the Lewis definition, because the proton transfer process is essentially the
reaction where the base uses its electron pair to accept a proton, as indicated by the mechanism arrow that we learned
earlier. Therefore in the following reaction, the BL acid, H
+
, is also the Lewis acid, and BL base, NH
3
, also fits to the
definition of the Lewis base.
Figure 3.5a Lewis base & Lewis acid reaction
However, the Lewis definition is broader and covers more situations. For the following reaction, B(CH
3
)
3
is the Lewis
acid because boron has an incomplete octet, and the empty 2p orbital on boron is able to accept electrons. (CH
3
)
3
N
behaves as the Lewis base with the lone pair electron on N that is able to be donated.
Figure 3.5b LA-LB adduct
The product between Lewis acids and Lewis bases is usually a species that has the acid and base joined together, and
the product is called the “LA-LB adduct”.
Other examples of Lewis acids include electron-deficient species, such as H
+
, M
+
, M
2+
, BH
3
, BF
3
, AlCl
3
etc. Lewis bases
can be: amine, ether or other species that have lone pair electrons to donate.
106 | 3.5 Lewis Acids and Lewis Bases

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Exercises 3.3
Show the product of the following LA-LB reaction:
Answers to Practice Questions Chapter 3
3.5 Lewis Acids and Lewis Bases | 107

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Answers to Practice Questions Chapter 3